Emmetropia describes an eye that lacks visual defects. This means the image that is formed on person’s retina is clear and precise. The eye with emmetropia doesn’t require contacts or eyeglasses. A person who has emmetropia in both eyes may be considered to have ideal vision, though it is sometimes described as perfect vision as well.

With emmetropia, a person’s eye is free of refractive error. When a person has a refractive error, light that enters the eye doesn’t bend correctly. The bending of light as it enters the eye is called refraction. Refractive error is the most commonly diagnosed type of vision problem. People with this error have abnormally shaped eyes that fail to bend the light properly, resulting in blurry vision. When emmetropia is present in both eyes, the eyes are shaped normally and refract light as expected.

To understand emmetropia fully, a person must have knowledge of how the eye works. Light is bent as it moves through a curved lens or though water. This is similar to the refraction of light that happens in the eyeball. Much of the eye's refraction happens as light moves through the cornea, which is the transparent covering on the front of the eye. The lens of the eye, the tear film on the eyeball, and the fluid in the eye all work to bend light as well.

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When light travels through the eye and is refracted, it is focused into a precise focal point in the center of the retina. The retina is the tissue lining the back of a person’s eye. Special retinal cells, called photoreceptors, capture the images a person sees and transmit the image details to the brain via the optic nerve.

If the eye is an abnormal length or the cornea is abnormally shaped, emmetropia isn’t possible. For example, if a person’s eye is longer than it should be, light is focused in front of the retina instead of on it. This causes a person to be nearsighted. If a person’s eyeball is too short, the images are focused behind the retina. In such a case, the affected person is said to be farsighted.

Sometimes visual problems are caused by a curved cornea. For example, if a person's cornea isn’t perfectly sphere-shaped, light isn’t focused on one retinal point. Instead, it focuses on two points, creating a condition called astigmatism. People with this condition often have corneas that are shaped like eggs or footballs. Astigmatism can affect one or both eyes and cause blurred vision.

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